As small consumer electronics such as mobile phones and personal digital assistants (PDAs) continue to proliferate at an unprecedented rate, the need for smaller and more reliable chip packaging has increased. In particular, a group of chip packages have been developed in which the chip, or die, is bonded to a package substrate through a number of solder bumps, rather than wire bonds. Such chip packages include, for example, flip-chip packages, direct chip attach packages, wafer-scale chip scale packages (CSPs), and fine-pitch CSPs.
Such packages often include an encapsulant, or underfill, material disposed generally between the die and the substrate, which increases the structural integrity of the package. In some applications, no-flow underfill materials are used in order to increase package throughput and reduce fabrication costs. No-flow underfill materials include flux, which cleans the solder bumps before the bumps are attached to the package substrate.
By using such a no-flow underfill material that includes flux, the underfill may be dispensed onto the substrate before the die is attached to the substrate, which eliminates the time-consuming process of separately heating the substrate to bond the solder bonds to the substrate and then dispensing the underfill material into the areas between the solder bonds. However, a common problem associated with the use of no-flow underfills is the presence of voids or bubbles formed within the underfill material. Such voids or bubbles negatively impact the structural integrity and reliability of the package. For example, moisture may becoming trapped inside such voids and later expand, thus causing the die to separate from substrate, a phenomenon referred to as “popcorn failure.” As another example, during subsequent heating processes (such as during a subsequent reflow process to attach the package substrate to a printed circuit board), solder material from solder bumps may flow into voids within the underfill material and cause a short between two or more solder bumps.